Be Positive
It is called motivational speaking for a reason.
Public Speakers eventually have to speak to an audience.
If an audience member is being negative or just plain mean, then act the same way as a stage magician would. Laugh it off. Say something witty. But do not fight with the audience or even with a single member as you will look worse merely for the fighting. The audience will always win. Instead, just let the audience member say what is desired, and ignore it or laugh it off or resond with a witticism. Eventually, the audience memeber will quiet down or leave and; then, your crisis is averted.
